The following methods have been implemented to manufacture molded resin products by a molding device for an injection molding machine, an injection compression molding machine, and an extrusion molding machine:
(1) A method by which resin is molded by moving a screw forwardly along a specified distance to inject resin and controlling a screw drive hydraulic pressure.
(2) A method by which a resin passage is closed and molds are pressurized by a press mechanism or the like, after moving a screw forwardly along a specified distance for injecting resin.
However, in the case of molding a product with a large projection area (that is, a large compression stroke), the molded products have had insufficient strength due to insufficient density or have had variable quality due to a number of flashes, because molded products vary substantially in weight despite the products being satisfactory in dimensional accuracy. In other words, there has been a problem including the following points:
(1) A hydraulic actuator is used to move the screw forwardly and its position is controlled in accordance with information fed back from a position sensor. However, it is difficult to accurately stop the screw at a specified position because of a time lag.
(2) It takes a certain amount of time to close a resin passage and, moreover, the amount of resin fed into a mold varies with the time necessary for closing the resin passage and any fluctuation of the screw position at which the resin passage begins to be closed, and therefore, it is difficult to maintain a stable weight of molded products.
An object of the present invention made in view of the above problem is to provide a device and a method of molding for an injection molding machine capable of reducing fluctuation in weight of resin molded products and of providing stable quality.